Upscaling linear chemical reactions in porous media; a case study of Dry Creek, ID

The exchange of surface and subsurface waters plays an important role in understanding and predicting large scale transport processes in streams and rivers. Faithfully representing the influence of small-scale physical and chemical processes associated with the subsurface on exchange is necessary for developing reliable upscaled predictive models at the reach scale and beyond. 2021 research introduced a novel continuous time random walk model to predict transport in an open channel system with hyporheic exchange and linear reactive processes in the subsurface. The methodology uses particle trajectory data from direct numerical simulations of a turbulent channel. We study the influence of chemical species' properties (via Damkohler numbers) and subsurface bed depth influence emergent large-scale transport behavior in the surface. The theory has been validated using direct numerical simulations (DNS), which is a high-fidelity modeling system to simulate natural environments. DNS solves the governing flow and transport physics equations; this research compared mathematical theory with DNS results to test theory robustness and gain a better understanding of the underlying physical processes that control large-scale river transport behavior. The next step of this project was to validate the theory in a real riverine system. This project will introduce tracers (NaCl) into a turbulent channel and characterize how the solute plume evolves in space-time. Conductivity sensors were used to measure solute concentration at 10m, 50m, 100m, and 250m from the solute injection-point. Results from the experiment will be compared to previously developed mathematical theory. The results of this experiment will lead to better understanding of how chemicals are transported in real-life systems, provide insights into residence time distribution of solute plumes, which can inform decision makers of optimal sustainable water management policies. Rivers and streams are important for transporting minerals, nutrients, and other chemicals throughout the natural environment. Introducing potentially toxic species into local water systems, e.g. agricultural-based pesticides, can create toxic chemical plumes that have negative ecological consequences